Why does Cisco PVST+ and Rapid PVST+ insert VLAN tag into extended system ID instead of 802.1q field?

[Sam Brynes]

At first I thought Cisco PVST+ and Rapid PVST+ just added an 802.1q VLAN tag, but it adds it into the extended system ID instead.

Why did they do this? Is it because 802.1q hadn't come out at that time yet?

[Jaderson Pessoa]

extendend system id is used in the root process.. for exemple:

vlan 10 = priority 32768 = 32768 + 10 = 32778
vlan 20 = priority 32768 = 32768 + 20 = 32788

[luis_cordova]

Hi @Sam Brynes 

I think this discussion gives some good explanations:
https://learningnetwork.cisco.com/thread/21718

Regards